International arms dealer Viktor Bout, the so-called “Merchant of Death” named in February in a federal grand jury indictment in New York on charges of conspiring to finance an aircraft fleet to arm bloody conflicts and support rogue regimes worldwide, will be returned to the United States to stand trial, a court in Thailand ruled on Friday.

In overturning a lower court’s decision that blocked his return, a Thai appeals court agreed with U.S. prosecutors that Mr. Bout, 43, should be returned to answer the accusations. He was being held in Thailand on a U.S. indictment handed up in 2008 accusing him of selling weapons to Marxist rebels in Colombia to kill Americans.

The U.S. government, which summoned the Thai ambassador in Washington this week to push for extradition, had considered the case as one of the nation’s highest priorities, according to the State and Justice departments.

“We are extremely pleased that the appeals court in Thailand has granted the extradition of Viktor Bout to the United States on charges of conspiring to sell weapons to a terrorist organization for use in killing Americans,” said Acting Deputy Attorney General Gary G. Grindler.

“We have always felt that the facts of the case, the relevant Thai law and the terms of our bilateral extradition treaty clearly supported the extradition of Mr. Bout on these charges,” he said, adding that while the Bout prosecution was “of utmost priority to the United States,” the criminal charges he faces are not solely an American concern.

“He has been sanctioned by the United Nations for alleged arms trafficking activity and support of armed conflicts in Africa,” Mr. Grindler said.

Justice Department officials based at the U.S. Embassy in Bangkok were in court to hear the verdict, and declined comment to the media. But Russian Foreign Minister Sergei Lavrov blasted the ruling, saying his country intended to have Mr. Bout returned to that country.

“This decision, according to the information that we have, was taken under very strong pressure from the outside. This is sad,” Mr. Lavrov said, referring to the United States . “I assure you that we will continue to do everything necessary to obtain his return to the motherland.”

Mr. Bout and an associate, Richard A. Chichakli, were accused of money-laundering, wire-fraud and of conspiring to purchase two aircraft from companies in the U.S. in violation of economic sanctions that prohibited such financial transactions.

According to the indictment, Mr. Bout carried out a massive weapons-trafficking business by assembling a fleet of cargo planes capable of transporting weapons and military equipment to various parts of the world, including Africa, South America and the Middle East.

The arms that Mr. Bout has sold or brokered have fueled conflicts and supported regimes in Afghanistan, Angola, the Democratic Republic of the Congo, Liberia, Rwanda, Sierra Leone and Sudan, the indictment said. He has controlled a large fleet of Soviet-era cargo aircraft since at least 1996, it said.

The indictment said that since the breakup of the Soviet Union, Mr. Bout was able to acquire surplus or obsolete aircraft he used to deliver arms and ammunition. Because of his extensive network of aircraft and operations companies, it said he had the ability to “transport large-scale military machinery, as well as extensive stores of weapons to virtually any location in the world.”

Because of U.N. sanctions against many of the aircraft companies they ran, the indictment said, Mr. Bout and Mr. Chichakli began in 2007 to form new companies to further their goal of transporting arms and ammunition worldwide and did so secretly by registering the companies in the names of others and hiding their involvement.

The indictment said they created and registered Samar Airlines in Tajikistan in the others’ names, and then set out to purchase airplanes on Samar’s behalf mainly a Boeing 727-200 and a Boeing 737-200. Nearly $2 million was transferred by wire into the U.S. from banks overseas to facilitate the purchase.

Mr. Bout also was indicted by a federal grand jury in New York in May 2008, accused of conspiring to sell to the Revolutionary Armed Forces of Colombia (FARC) millions of dollars worth of weapons, including surface-to-air missile systems, armor-piercing rocket launchers, AK-47 firearms, millions of rounds of ammunition, anti-personnel land mines, C-4 plastic explosives, night-vision equipment, ultralight airplanes that could be outfitted with grenade launchers and missiles and unmanned aerial vehicles.

He was arrested by Thai authorities on a provisional arrest warrant, based on a complaint filed in the U.S. District Court for the Southern District of New York, charging conspiracy to provide material support or resources to a designated foreign terrorist organization.

According to the first indictment, during a covertly recorded meeting in Thailand on March 6, 2008, Mr. Bout told undercover agents he could arrange to airdrop the arms to the FARC in Colombia, and offered to sell to the FARC two cargo planes that could be used for arms deliveries. That indictment said Mr. Bout told the agents he understood they wanted to use the arms against American personnel in Colombia, adding that the U.S. also was his enemy and that the FARC’s fight against the U.S. also was his fight.

Mr. Bout is a Russian who formerly served as a military translator but has been in the arms business since the mid-1990s. Mr. Chichakli, 51, is a U.S. citizen who was born in Syria and served in the U.S. Army. They have been close business associates for the past several years.